Refractory substance

ABSTRACT

A dead-burned refractory magnesia composition suitable for use in the manufacture of high hot strength magnesia refractories comprises at least 95% by weight MgO, less than 0.4% by weight of silica, an amount of lime greater than (2S + 0.3)% by weight, where S is the percentage by weight of silica, the weight ratio of lime to silica in the composition being greater than 3:1, and boron in an amount of less than 0.25% by weight expressed as B2O3.

United States Patent [1 1 Spencer Nov. 6, 1973 REFRACTORY SUBSTANCE3,364,043 l/1968 Davies 106/58 [75] Inventor: Denzil Reginald FrankSpencer,

Oldcotes, near Worksop, England Primary ExaminerDelbert E. Gantz [73]Assignee: Steetley (Mfg.) Limited, Worksop, Assis'am Examiners- BergerEngland AttorneyBac0n & Thomas [22] Filed: Oct. 8, 1971 [2]] Appl. No.:187,906 [57] ABSTRACT A dead-burned refractory magnesia compositionsuit- 1 Forelgn Apphcaum P nomy able for use in the manufacture of highhot strength Oct. 9, 1970 Great Bl'ltam 48,176/70 magnesia refractoriescomprises at least 95% y ig MgO, less than 0.4% by weight of silica, anamount of UOSI Cll a. li g t h ight, where S i th [51] 'f Cl C04b 35/04percentage by weight of silica, the weight ratio of lime [58] .Fleld 0fSearch 106/58, 61 to Silica in the composition being greater than3:1Land boron in an amount of less than 0.25% by weight ex- [56]References Cited pressed as B2O3 UNITED STATES PATENTS 3,378,383 4/1968Yan Dreser 106/58 2 Claims, No Drawings The present invention relates torefractory materials and in particular to refractory grade magnesiacompositions and to refractory products such as refractory bricks whichare made therefrom.

It is at the present time a widely held belief in the refractoryindustry that, for any given quality of magnesia, the lower the boroncontent the greater the high temperature strength of the grain or thebricks made from such magnesias. It has now been found however that formagnesias having silica contents of less than 0.4% by weight andcontaining lime and silica at lime/- silica weight ratios greater than3:1 boron contents sometimes as high as 0.25% by weight B can betolerated while maintaining comparatively high strength at elevatedtemperatures.

Accordingly the present invention provides a deadburned refractorymagnesia composition comprising at least 95% by weight MgO, preferablyat least 97% by weight MgO, less than 0.4% by weight of silica,preferably from 0.005 to 0.02% by weight of silica, an amount of limegreater than (28 +-0.3)%by weight, preferably greater than (23 0.5 byweight, where S is the percentage by weight of silica, the weight ratioof lime to silica in the composition being greater than 3:1 and boron inan amount of less than 0.25% by weight expressed as 3,0,.

While quite significant levels of B 0 can be tolerated in the magnesiacomposition of the present invention, it has to be appreciated thatthere is some falling off in high temperature strength at silicacontents in the region of about 0.3% by weight with B 03 levels at about0.2% and it is preferred in order to obtain good strength at the highersilica contents to limit the B 0 content to less than about 0.1%. Ingeneral the minimum B 0 need only be about 0.258 and the maximum B 0 maybe about (0.25 1.28)%, where S is the percentage weight silica. Someadvantage may be obtained by operating at a 13 0 level below (0.30.8S)%.

Iron oxide and alumina may be present in the composition of the presentinvention but the amount of these materials is preferably maintained ata minimum. For example the amount of iron oxide and alumina is less than0.4% by weight, preferably less than 0.2% by weight, of R 0 where R 0represents the total weight of Fe 0 and A1 0 In general the amount of R0 materials which can be tolerated decreases as the amount of limepresent increases and R 0 is desirably of the order of 0.1% by weight athigh lime contents e.g. at about 2% CaO or more.

The refractory composition of the present invention may be obtained frommagnesia-containing minerals but is preferably obtained from syntheticmagnesias i.e. magnesias which have been precipitated from bitterns,

brines or sea water by treatment thereof with alkali or which havebeenobtained by thermal decomposition of magnesium chloride. Thermaldecomposition of magnesium chloride is preferably carried out by themethod described in British Patent Specification No. 793700 whichcomprises the steps of disintegrating the magnesium chloride in themolten state or as an aqueous solution into a suitable directed spray ofpredetermined length of path, contacting the spray with a stream of hotgas in the presence of water-vapour and thermally decomposing thesprayed particles before they reach the end of said path. In order toobtain the particular composition of the present invention it may benecessary to purify and/or tomake additions of for example chrome-,lime-, or zirconia-bearing materials to the magnesia and/or to modifythe conditions under which the magnesia is formed. The magnesia will beone A which has been dead-burned or which has been pelletised anddead-burned.

The refractory compositions of the present invention with or withoutother refractory materials may be made into bricks, blocks, cements,ramming compositions gunning mixes or castables and these may be used infurnace constructions. They are characterized by good density andstrength at high temperatures.

The following examples are given to illustrate the present invention.

EXAMPLES l to 8 A series of sea water magnesias of varying analyses werelightly calcined at a temperature in the range 600 to l,000 C. until themagnesia had between 0.5% and 2% ignition loss. The contents of theimpurities lime, silica, iron oxide and aluminum oxide were thenadjusted to the required levels by adding an appropriate amount ofmaterial, for example, silica, lime, zircon and zirconia, chrome, etc.,and mixing thoroughly. The mixture was then pelletised at 20 tons persquare inch and dead-burned at l,500 C. to l,800 C.

The dead-burned magnesias were made into bricks by the following method.The magnesias were crushed and graded into a batch suitable forbrick-making containing of particles between 5 and 72 8.8.8. mesh and35% of particles passing a 72 8.8.8. mesh. Approximately 4% of sulphitelye solution of specific gravity 1.2% was added to the mixture pressedin a mould at 8 tons per square inch. The resulting bricks were fired inthe range of l,650 to 1,800 C. over a 112 hour schedule.

Modules of rupture was determined on the bricks at various temperatures.The results are set out in Table I from which it will be seen that eachof the exemplified magnesia compositions from which the bricks were madepossessed analyses within the scope of the present invention and eachbrick was shown to have comparatively strength at elevated temperatures.

TABLE 1 Example No.

Ma nesia analysis percent:

CaO 0.9 1.41 1.38 1.2 1.2 0.9 1.0 0.85 SiOg............ .l 0.08 .25 0.30.3 .2 0.1 .04 A1203 .05 .07 .03 .2 .2 .2 .04 .05 Fe:O;;......... .l .1.12 .15 .15 .15 1.1 .95 Cr O .3 .3 ZrO .4 .3 B20 2 .l .16 .02 .04 .03 1.16

Modulus of rupture p.s.i. at:

l350C.... 1,100 1400c.... 950 750 1,300 1500C.... 850 650 1.150 1,000

1550c.... 900 l600C 750 850 COMPARATIVE EXAMPLES 2 1% 0.1 .1 0.12 0.15061;

'2 By way of comparison, the modules of rupture of 0.3 bricks having alime to silica ratio below 3:1 was meas 'g of Rupture 02 (H8 016 suredusing the same techniques as used in Examples 1 psi at to 8. Theformulations used and the results obtained 250 220 350 700 500 are givenin Table 2 as Comparative Examples a to e. 400C 250 The results obtainedindicate that bricks made from I claim magnesias having similar analysesto those used in Examples l to 8 but with lower lime to silica ratioshave much lower strengths.

l. A dead-burned refractory magnesia composition comprising at least byweight MgO, from 0.005% to 0.2% by weight of silica, an amount of limegreater than (28 0.5)% by weight, where S is the percentage by weight ofsilica, and boron in an amount of 0.1% to 0.25% by weight expressed as B0 2. A dead-burned refractory magnesia composition according to claim 1comprising at least 97% by weight MgO.

2. A dead-burned refractory magnesia composition according to claim 1comprising at least 97% by weight MgO.